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2017 O Level Chem P1 Q15 — Energy Changes / Enthalpy of Combustion Solution

Energy Changes / Enthalpy of Combustion 2017 O Level Pure Chemistry · Paper 1 · Q15

Question

The enthalpy change of combustion, ΔH, of an alcohol is the heat given out when one mole of the alcohol is completely burned in oxygen. The enthalpy change of combustion for some alcohols is given. How many carbon atoms and how many hydrogen atoms would there be in the formula of an alcohol that has an enthalpy change of combustion of −3900 kJ/mol? Question diagram

Question diagram

Question diagram (continued)

A
B
C
D

Correct answer: D

Video Solution

Explanation

Answer: D. Each extra \(\text{CH}_2\) in the chain releases a further 630 kJ/mol on combustion (the values rise by 630: 750, 1380, 2010, 2640). Continuing the pattern gives C5 = 3270 and C6 = 3900.

So an enthalpy change of \(-3900\) kJ/mol is a six-carbon alcohol, \(\text{C}_6\text{H}_{13}\text{OH}\), which has 6 carbon atoms and 14 hydrogen atoms.

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